Ring oscillators generally include an odd number of series coupled stages, and provide a periodic output signal. Generally, the frequency of the output signal depends, among other things, on the rate at which each of the stages charges and discharges its respective output node (which is the respective input node for the next stage in the series). For example, if each stage includes an inverter formed from an n-channel field effect transistor (nFET) and a p-channel field effect transistor (pFET), the frequency of the output signal generally depends on the rate at which each nFET discharges that stage's output node and the rate at which each pFET charges that output node. The rate at which the nFET and pFET respectively discharge and charge the output node in turn depends, among other things, on the magnitude of the supply voltage provided to the inverters and the operating temperature of the oscillator. Generally, the lower the operating temperature and the higher the magnitude of the supply voltage, the faster the nFETs and pFETs will respectively discharge and charge the output node due to the nFETs and pFETs being more conductive and less resistive when respectively discharging and charging the output node. The faster that the nFETs and pFETs respectively discharge and charge the output node, the shorter the delay through each stage, and consequently, the faster the frequency of the periodic signal generated by the ring oscillator.
Conventionally, the magnitude of the supply voltage for the stages in a ring oscillator is regulated in order to reduce variations in oscillation speed for different supply voltage magnitudes. For example, if the external supply voltage provided to the oscillator is known to vary between 1.28V and 1.575V, a supply voltage regulator may provide a regulated voltage of approximately 1.35V to the oscillator regardless of changes in the external voltage provided. However, merely regulating the supply voltage with a supply voltage regulator does not remedy variations in the charging and discharging times for each stage caused by temperature variations. Also, the regulators that regulate the supply voltage can occupy space on and consume power in an integrated circuit.
If the supply voltage regulator is removed, however, the variations in oscillation speed for different supply voltage magnitudes can be relatively large and unpredictable. For example, if a voltage regulator does not regulate the supply voltage provided to the stages, an increase in the magnitude of the supply voltage will generally increase the frequency of the output signal generated by the ring oscillator, as described above.